Bridge amplifiers are useful in audio power amplifiers and in control circuits for delivering bidirectional current to an output load device.
A typical bridge amplifier comprises first and second amplifiers arranged for delivering power to a load device. The load device is connected between the output of the first amplifier and the output of the second amplifier. First and second drive signals, 180 degrees out of phase with each other, are supplied to the respective input terminals of the first and second amplifiers.
Compared to single-ended amplifiers, bridge amplifiers can provide twice the output signal voltage across a load device for the same power supply voltage. This results in four times the power output being delivered to the load. For such reason, bridge amplifiers are particularly useful in automotive or aircraft applications where available battery voltage is limited.
A bridge amplifier employing field-effect transistors (FET's) is described in U.S. Pat. No. 4,117,415, issued to the present applicant and assigned to RCA Corporation. The use of FET's is advantageous in that such devices provide high input impedance, wide dynamic range, square law transfer characteristics for low distortion, relative freedom from thermal runaway, and freedom from secondary breakdown characteristics.
FIG. 1 of U.S. Pat. No. 4,117,415, cited above, shows a bridge amplifier circuit using three pairs of field-effect transistors, each pair arranged as a complementary amplifier configuration, to drive a load device in response to a single-ended input signal. Each complementary-symmetry FET amplifier includes a P-channel FET and an N-channel FET. The respective drain electrode of each FET is connected to a respective output terminal; the respective gate electrode of each FET is connected to a respective input terminal; and an operating potential is impressed across the respective source electrodes. In the bridge amplifier circuit, the first and second pairs of field-effect transistors are arranged as first and second amplifiers for driving respective ends of the load device. The third pair of field-effect transistors serves as an inverting amplifier. The single-ended input signal is connected to drive the input of the first and third amplifiers and the output of the third amplifier is connected to drive the input of the second amplifier. The present invention is directed towards eliminating the third amplifier which provides 180 degrees of phase shift in bridge circuit configurations.